Process for preparing perfluorotriazine elastomers and precursors thereof

ABSTRACT

Perfluoroether triazine elastomers having improved properties and utility in seals, gaskets, sealing components and the like are prepared from oligomeric imidoylamidines that have, in turn, been prepared by the process of (a) reacting a perfluorodinitrile with liquid ammonia to yield a perfluorodiamidine, (b) isolating the perfluorodiamidine, (c) reacting the isolated diamidine with a perfluorodinitrile to yield a perfluoro(imidoylamidine) dinitrile, and then repeating steps (a), (b), and (c) to sequentially grow an oligomer of desired molecular size. The isolated amidine and nitrile intermediates are also disclosed.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

TECHNICAL FIELD

This invention is in the field of perfluoroalkylether triazines andperfluoroalkyl triazines and elastomers made therefrom. Moreparticularly, the invention relates to an improved process for preparingsuch materials.

BACKGROUND

The perfluorotriazine polymers to which the present invention relatesare, as a general class, known materials. See U.S. Pat. No. 4,242,498issued Dec. 30, 1980 to Rosser, et al. These materials are tough, heatand chemical-resistant elastomers. These advanced properties allow theiradvantageous use in extreme environments and applications--for exampleas fuel tank sealants, O-rings, wire enamels, pneumatic ducts and edgeclose-outs in aircraft.

U.S. Pat. No. 4,242,498 discloses a 4-step preparation procedure forthese elastomers. This procedure consists of (1) forming apoly(imidoylamidine) by the reaction under reflux conditions ofanhydrous ammonia with certain perfluorinated alkyl or alkyletherdinitriles; (2) forming a linear polytriazine by cyclizing theimidoylamidine linkages by reaction with certain perfluorinated alkyl oralkylether acid anhydrides or halides; (3) extending the linearpolytriazine chain by further refluxing in anhydrous ammonia; and (4)heating to cyclize the new imidoylamidine linkages and thereby crosslinkthe polymer.

While this earlier process and the products it produced were good usefuladvances, they did pose shortcomings. For one, with this process, it wasdifficult to control or reproduce the molecular size of the polymerproduct. Moreover, the resulting polymer at times manifested some degreeof hydrolytic instability.

Other patents that are of general interest in showing various othersimilar procedures for producing triazine elastomers of the general typedescribed above include:

U.S. Pat. No. 3,347,901 issued on Oct. 17, 1967 to Fritz, et al;

U.S. Pat. No. 3,250,808 issued on May 10, 1966 to Moore, et al;

U.S. Pat. No. 3,450,646 issued on June 17, 1969 to Annand, et al;

U.S. Pat. No. 3,531,496 issued on Sept. 29, 1970 to Annand, et al;

U.S. Pat. No. 3,637,630 issued on Jan. 25, 1972 to Dorfman, et al;

U.S. Pat. No. 4,234,715 issued on Nov. 18, 1980 to Rosser, et al;

U.S. Pat. No. 4,245,085 issued on Jan. 13, 1981 to Rosser, et al; and

U.S. Pat. No. 4,273,918 issued on June 16, 1981 to Rosser, et al.

STATEMENT OF THE INVENTION

It has now been found that more uniform and reproducibleperfluoroalkylether and perfuoroalkyl triazine elastomers result whencertain improvements are made to the art-taught preparation process.

The process of this invention is a multistep process in which first aperfluorodinitrile is reacted with liquid ammonia to give aperfluorodiamidine which is isolated. This material is then reacted withadditional perfluorodinitrile to give aperfluoro(imidoylamidine)dinitrile. These two steps are then repeated,that is, this new dinitrile is then reacted with liquid ammonia to givethe corresponding perfluoro(imidoylamidine)diamidine which is freed ofammonia and is in turn reacted with additional dinitrile (either theoriginal dinitrile, another dinitrile, or the imidoylamidinedinitrile)in a stepwise growth. This two step sequence can be repeated until alinear oligomer having the molecular size desired is achieved. Then thelinear oligomer is cyclized with a triazine cyclizing reagent,preferably with a perfluoroacid anhydride to give an oligomericperfluoro(triazine)dinitrile prepolymer which can be crosslinked to givean elastomer by heating in the presence of ammonia.

Expressed most directly, in the process of the invention, the priorart's reaction of ammonia and a perfluorodinitrile to give a polymer isnewly broken down into its individual reaction steps by adoption ofparticular reaction conditions. These steps are repeated sequentiallyuntil the desired size product is reproducibly achieved.

In addition to this process, with or without the final cyclizing andcrosslinking steps, this invention also provides the perfluorodiamidinesand the poly(imidoylamidine)dinitriles as new chemical compounds.

DETAILED DESCRIPTION OF THE INVENTION The Reaction Sequence

In particular, this new process involves (A) reaction aperfluorodinitrile NC--(R_(f))--CN, wherein R_(f) is either a bivalentfluorocarbon radical of the formula --C_(p) F_(2p) --, wherein p is anumber from 2 to 18 or a bivalent oligomeric oxyfluorocarbon radical ofthe formula

    --CFY(OCF.sub.2 CFY)--O--(CF.sub.2)--O--(CFYCF.sub.2 O)--CFY--

wherein Y is F or CF₃, q is a number from 1 to 18 and m and n are eachnumbers the sum of which is from 2 to 7, with a molar excess of liquidammonia to form a perfluorodiamidine; ##STR1## isolating theperfluorodiamidine; (C) reacting this isolated diamidine with additionalperfluorodinitrile to produce an oligomeric (imidoylamidine)dinitrile##STR2## wherein n is a number initially 2, but ultimately from 2 to 50inclusive, preferably 2 to 10 inclusive and more preferably 2 to 5. This(imidoylamidine)dinitrile is then (D) reacted as in step (A) with amolar excess of liquid ammonia to produce an (imidoylamidine)diamidine##STR3## which is (E) isolated as in step (B) and (F) reacted with anappropriate dinitrile as in step (C). This growth is continued until thedesired molecular size is attained at which time the nitrile is freed ofunreacted ammonia and (G) reacted with a ring-closing reagent,especially a perfluorinated acid anhydride, (R_(f) 'CO)--O, whereinR_(f) ' is a monovalent fluorocarbon radical or oxyfluorocarbon radical,to produce a perfluoro(triazine)dinitrile ##STR4## Thereafter, thisdinitrile prepolymer may be cured (crosslinked) to an elastomer byheating in the presence of added ammonia.

Feedstocks

The perfluorodinitrile feedstock, NC--(R_(f))--CN, may be selected frompreferably linear but also branched fluorocarbon dinitriles, NC--C_(p)F_(2p) --CN, wherein p is a number from 2 to 18, preferably 5 to 12.These materials are known and may be prepared from the correspondingdiacids which are marketed commercially by PCR Research Chemicals, Inc.The NC--(R_(f))--CN feedstock may also be selected from oligomeric(perfluoroalkylether)dinitriles,

    NC--CFY(OCF.sub.2 CFY)--O--(CF.sub.2)--O--(CFYCF.sub.2 O)--CFY--CN,

wherein Y is F or a trifluoromethyl group (--CF₃), q is a number from 1to 18 inclusive (preferably 1-5, most preferably about 3), m and n areeach numbers from 1 to 5 inclusive with the sum of m+n equal to 2 to 7.Preferably m and n are equal and are each 2 or 3. These materials areknown and can be prepared as taught in U.S. Pat. No. 3,347,901 of Fritz,et al, which disclosure is incorporated herein by reference. Many ofthese materials are also available from PCR, Inc. R_(f) represents thebivalent perfluorinated radicals which correspond to these dinitriles.Mixtures of two or more dinitriles may be used, as well.

The cyclizing agent feedstock is selected from the anhydrides, acylhalides (e.g. fluorides and chlorides) of perfluorinated lower(preferably branched, but also linear) aliphatic acids containing from 2to 18 carbons and of the oligomeric perfluoroalkylether acids. Theseacids and their corresponding cyclizing agents are represented by theformulae ##STR5## respectively. wherein R_(f) ' is a linear or branched1 to 17 carbon (preferably branched 1 to 11 carbon) monovalentperfluoroalkyl radical of the formula --C_(r) F_(2r+1), wherein r isfrom 1 to 17 (inclusive preferably 1 to 11) or an oligomericperfluoroalkylether of the formula

    --CF.sub.2 Y--CF.sub.2 O(CFY--CF.sub.2 O)--CFY--

wherein Y is F or a perfluoroalkyl group of 1 to 3 carbons especially For CF₃, and y is an integer from 1 to 50 inclusive, preferably 1 to 10and more preferably 1 to 5. Of these cyclizing reagents, the oligomericperfluoroalkylether anhydrides, especially wherein Y is CF₃, and y isfrom about 1 to 3 inclusive are preferred. With these oligomericmaterials, a mixture of chain lengths is often most easily obtained andmay be used as may other mixtures. The perfluoro lower aliphatic acidsare commercially available and their anhydrides and halides may beprepared by methods of the art. The perfluoroalkylether acids may beprepared by first making the corresponding acid fluorides according tothe procedures of Moore et al's U.S. Pat. No. 3,250,808, by hydrolyzingto the free acids and then dehydrating such as with P₂ O₅.

Reaction Conditions The Diamidine Formation Reaction

This reaction, whether carried out using the starting material dinitrile(i.e. Reaction A) or a dinitrile intermediate formed in the reactionsequence (i.e. Reaction D), is carried out in the presence ofsubstantial excess liquid ammonia. Generally this is achieved by a slow,e.g. dropwise, addition of the dinitrile to the liquid ammonia withvigorous agitation. Preferably at least 10 and especially 15 to 200equivalents of ammonia are present per equivalent of dinitrile. Thisreaction is carried out in liquid phase at temperatures at or below theboiling point of ammonia, generally at temperatures of from -60° to-33°, preferably -50° to -30°. If super-atmospheric pressures are usedto assure a liquid ammonia reaction phase, higher temperatures such asup to 50° C. can be used. The reaction times will vary between 0.1 and24 hours, inversely depending upon the reaction temperature. Longertimes may be used but are not required. If a solvent is present, such asas a vehicle for the dinitrile, it should be a material that dissolvesthe reactants and is inert to them, with liquid fluorocarbons,especially the fluorocarbons marketed as FREON®, and more especiallyFREON® 113 (C₂ Cl₃ F₃) being preferred.

Isolation B

This isolation step is employed to substantially free theamidine-forming reaction mixture of step A or of subsequent steps ofammonia and any added solvent. Preferably, the product of this isolationshould contain as little ammonia as possible i.e. it should contain nomore than 1% by weight of ammonia, more preferably less than 0.1% byweight NH₃. In view of the volatility of the solvent and NH₃, vacuum isa preferred method for their removal. Other equivalent means may beemployed.

The Diamidine-Dinitrile Condensation C, etc.

This reaction (reaction C, F, etc.) is carried out substantially in theabsence of ammonia. One reactant is slowly admixed with the other atmoderate temperature, in liquid phase generally in an inert reactionsolvent. Preferred temperatures are from 0° C. to 70° C. withtemperatures of from 5° C. to 60° C. being preferred. The reaction timeranges from 1 hour to 72 hours with, as a guideline, 24 to 48 hoursbeing preferred at room temperature. The mole ratio of the two nitrileand amidine reactants is generally controlled at about 2.1 i.e. 1.5:1 to2.5:1. Higher and lower ratios may be used as well but can lead to finalproducts having differing properties. Also preferably, the amidine isadded to the nitrile. The inert solvents discussed in step A may besuitably used in this step as well.

The Ring Closure G

The reaction of the linear oligomeric dinitrile with the ring closuringagent to yield an oligomeric prepolymer is carried out in thesubstantial absence of ammonia. Generally, an inert liquid reactionsolvent is present, particularly the FREONs® discussed in Step (A). Amolar excess, i.e. greater than 1 and especially from 4 to 20equivalents of ring closing agent is usually employed. Time andtemperature for the reaction depend upon the type of ring closing agentemployed. With anhydrides, moderate temperatures such as from 0° to 70°C. for 1 to 3 hours are usually adequate. With acid halides, strongerconditions are called for as from room temperature to 100° C. for 12 to36 hours.

The Crosslinking Reaction

Crosslinking is achieved by heating the oligomeric prepolymer withammonia. Generally, a temperature of from 100° to 300° C. is employedfor from 1 to 200 hours, especially 100° to 200° C. for from 12 to 100hours. The amount of ammonia ranges from 0.1 to 4 equivalents perprepolymer molecule.

Products

The linear oligomeric prepolymer material ##STR6## can range inmolecular size from about 1500 daltons (when X equals 1 and R_(f) andR_(f) ' equals the preferred perfluoro ethers) up to 10,000 daltons orhigher such as up to 150,000 daltons. In other words X can range from 1to 100, if desired. It has been found, however, that very excellentproperties are imparted to the final elastomers when X is from about 3to about 10, especially from 4 to 8 and most especially such as to givea molecular weight of 6000-6500.

The final crosslinked elastomers are characterized by uniformproperties--especially uniform thermal, physical and chemicalproperties. They have a glass transition temperature of from -75° C. to-30° C. and exhibit less than 5% weight loss when heated in nitrogen orair for 3 days at 300° C. In addition, they resist hydrolytic attack.These final materials are excellent high performance elastomers and canbe fabricated into a variety of seals, gaskets, sealing components andlike by methods known in the art. Fillers and/or reinforcing agents canbe added, if desired.

The invention will be further illustrated by the following Examples.They are presented to make clear the invention and are not to beconstrued as limiting its scope.

EXAMPLES

In these Examples, all temperatures are in °C., and all weights are in gunless otherwise noted. Infrared (IR) spectra were recorded on a NicoletMX-1, FT-IR Spectrometer. Thermogravimetry measurements were recordedwith DuPont Instruments, the 951 thermogravimetry analyzer and the 990thermal analyzer. Glass transition temperatures (tg) were obtained witha DuPont differential scanning calorimetry (DSC) cell and 1090 thermalanalyzer. Molecular weight values were determined by gel-permeationchromotography measurements using a Waters Associates ALC-GPC 202/401liquid chromatograph equipped with a Spectra-Physics SP 4020 datainterface and SP 4000 central processor/plotter. The SP system wasprogrammed to correct for baseline and to compute the average molecularweight by comparison with known calibration points. Separations wereobserved by using DuPont size-exclusion (SE) columns having a WatersAssociates differential UV detector at 254 mm or by a differentialrefractometer R 101 is FREON® 113 solution according to the method ofKorus and Rosser, Anal.Chem., 50, 249 (1978). Gas chromatography resultswere obtained with a Hewlett-Packard Model 5830 gas chromatograph.

The 1,1,2-trichloro-1,2,2,-trifluoro ethane (FREON® 113) was purchasedfrom Fisher Scientific Company and distilled, using a 4-ft column of6-mm porcelain berl saddles at 48° C. Perfluoroalkylether dinitrileswere purchased from PCR Inc., and hexafluoropropylene epoxide wasobtained from E. I. DuPont de Nemours & Co. Other chemicals were eitheranalytical grade or better.

EXAMPLE 1

Ring Closing Agent Preparation: Preparation of Perfluoroalkylether AcidFluoride and Its Corresponding Anhydride.

Mixed perfluoroalkylether acid fluorides (I) (R_(f) 'COF) were preparedaccording to the procedure shown in U.S. Pat. No. 3,250,808 of Moore andMilian. ##STR7## Hydrolysis of the acid fluoride mixture gave thecorresponding perfluoroalkylether acids. The synthesis of the anhydridesof these perfluoroalkylether acids was performed by placing 200 g of themixed perfluoroalkylether acids and 160 g of phosphorous pentoxide in areaction flask; the mixture was heated to 180° C. and held at thattemperature for 16 hr and then vacuum distilled. 150 g of thecorresponding mixed perfluoroalkylether acid anhydride (II) wascollected (85° C./16 mm Hg-120° C./0.5 mm Hg).

    (R.sub.f 'CO)--O                                           (II)

The mixture was used without further purification.

Analytical data (IR liquid film): 1870,1805 cm⁻¹ (C═O), 1400-1050 cm⁻¹(C--F).

A. Preparation of Perfluoroalkylether Diamidine ##STR8## wherein R_(f)represents ##STR9## Twenty milliliters of NH₃ were condensed into a100-ml, three-neck flask equipped with a dry-ice condenser, an NH₃ gasinlet, and an addition funnel. A solution of 10 g of perfluoroalkyletherdinitrile NC--R_(f) --CN in 10 ml of FREON® 113 was added dropwise intothe flask with vigorous stirring.

B. After completion of the addition, excess ammonia and FREON® 113 wereremoved by vacuum. Liquid perfluoroalkylether diamidine (10.5 g) IIIsubstantially free of ammonia was then collected without furtherpurification.

Analytical data (IR liquid film): 3200-3000 cm⁻¹ (NH); 1684 cm⁻¹ (C═N);1400-1050 cm⁻¹ (C--F).

C. Preparation of Perfluoroalkylether (Imidoylamidine)--Dinitrile (IV)##STR10##

To a solution of 20 g of the perfluoroalkylether dinitrile NC--R_(f)--CN used in step A in 20 ml of FREON® 113 the solution of 10 g ofperfluoroalkylether diamidine (III) in 10 ml of FREON® 113 was addeddropwise with vigorous stirring for 2 hours at room temperature. Afterevaporating the solvent, 30 g of perfluoroalkylether(imidoylamidine)dinitrile (IV) was recovered as a viscous liquid.

Analytical data (IR liquid film): 3500, 3400, 3120 cm⁻¹ (N--H), 2260(--C.tbd.N), 1652, 1602, 1520 cm⁻¹ (imidoylamidine) 1400-1050 cm⁻¹(C--F).

D. Preparation of Perfluoroalkylether (Imidoylamidine)--Diamidine (V)##STR11## Perfluoroalkylether (imidoylamidine)dinitrile (IV) (30 g in 30ml of FREON® 113) was added dropwise to 30 ml of liquid ammonia withvigorous stirring for 3 hr at room temperature. Excess ammonia andsolvent were then removed under vacuum and 30 g of perfluoroalkylether(imidoylamidine)diamidine (V) was obtained as a viscous liquid withoutfurther purification. This material could be reacted with aperfluorodinitrile such as (IV) or NC--R_(f) --CN or the like as shownto give a higher molecular weight dinitrile.

E. Preparation of Linear Perfluoroalkylether (Triazine) Dinitrile(VI)--Prepolymer ##STR12## To 50 g of perfluoroalkylether acid anhydride(II) was added 25 g of perfluoroalkylether (imidoylamidine)dinitrile(IV) in 30 ml of FREON® 113 with stirring for 2 hours. After removingthe solvent, perfluoroalkylether acid, and excess perfluoroalkyletheracid anhydride by vacuum distillation, 28 g of perfluoroalkylether(triazine) dinitrile (VI) prepolymer was obtained as a highly viscousliquid.

Analytical data (IR liquid film): 2260 cm⁻¹ --(--C.tbd.); 1550 cm⁻¹(triazine); 1400-1050 cm⁻¹ (C--F).

F. Preparation of Cross-Linked Perfluoroalkylether Triazine Elastomer

Thirty-five grams of a perfluoroalkylether (triazine) dinitrile having a(Mv 6200) were stirred with 1-2 equivalents of liquid ammonium and thenheated in an open space in an oven at 150° C. for 4 days to effectcrosslinking. The resulting product (35 g) was a light tanperfluoroalkylether triazine elastomer with excellent properties. Theglass transition temperature of the elastomer, determined bydifferential thermal analysis was about -45° C., and isothermal weightlosses in air or nitrogen at 300° C. for 3 days were less than 5%.

Analytical data (IR liquid film): 1500 cm⁻¹ (C--F).

EXAMPLE 2

The preparation of Example 1 is repeated substitutingperfluoromethylacetic acid anhydride as the triazine ring closure agentin step E. The resulting triazine prepolymer can be crosslinked to givean elastomer. This product is somewhat less hydrolytically stable thanthe product of Example 1.

EXAMPLE 3

The preparation of Example 1, parts A,B,C and D, is repeated. Thediamidine (V) of part D (85 g), as an ammonia-free liquid is dissolvedin 10 ml of FREON 113 and added dropwise to a vigorously stirredsolution of 10 g of perfluoroalkylether (imidoylamidine)-dinitrile IV ofpart C of example 1. The mixture is stirred at room temperature for 2hours. After evaporating the solvent, a high molecular weightperfluorodinitrile of the formula ##STR13## is recovered. this materialis dissolved in 30 ml of Freon and then converted to a linear triazineprepolymer by the method of Example 1 part E, i.e. prepolymer byaddition to 50 g of anhydride II, and stirring for two hours. Thisyields the triazine. ##STR14## which can be cross-linked to give anelastomer in accord with Step F of Example I.

We claim:
 1. In the process for preparing an oligomericperfluoro(imidoylamidine)dinitrile by reacting an originalperfluorodinitrile selected from the group of materials having theformula NC--R_(f) --CH wherein R_(f) is selected from bivalentfluorocarbon radicals of the formula --(C_(p) F_(2p))--, wherein p is anumber from 2 to 18 inclusive, and bivalent oxyfluorocarbon radicals ofthe formula

    --CFY(OCF.sub.2 CFY)--O--(CF.sub.2)--O--(CFYCF.sub.2 O)--CFY--

wherein Y is F or CF₃, q is a number from 1 to 18 and m and n are eachnumbers, the sum of which is from 2 to 7 with ammonia, the improvementcomprising conducting the reaction in two steps, a first step comprisingreacting at least 10 equivalents of liquid ammonia and said originalperfluorodinitrile at a temperature of from -60° C. to -33° C. for from0.1 to 24 hours to yield a perfluorodiamidine and unreacted ammonia,and, following removing at least 99% of said excess ammonia from saidperfluorodiamidine, a second step comprising reacting saidperfluorodiamidine with from 1.5 to 2.5 equivalents of said originalperfluorodinitrile or another perfluorodinitrile selected from the samegroup of materials for 1 to 72 hours at 0° C. to 70° C. to yield aperfluoro(imidoylamidine)dinitrile.
 2. The process of claim 1 whereinR_(f) is a bivalent oxyfluorocarbon radical of the formula

    --CFY(OCF.sub.2 CFY)--O--(CF.sub.2)--O--(CFYCF.sub.2 O)--CFY--

wherein Y is F or CF₃, q is a number from 1 to 18 and m and n arenumbers, the sum of which is from 2 to
 7. 3. A process for preparing aperfluoro(imidoylamidine)diamidine which comprises(a) reacting anoriginal perfluorodinitrile selected from the group of materials havingthe formula NC--R_(f) --CH wherein R_(f) is selected from bivalentfluorocarbon radicals of the formula --(C_(p) F_(2p))--, wherein p is anumber from 2 to 18 inclusive, and bivalent oxyfluorocarbon radicals ofthe formula

    --CFY(OCF.sub.2 CFY)--O--(CF.sub.2)--O--(CFYCF.sub.2 O)--CFY--

wherein Y is F or CF₃, q is a number from 1 to 18 and m and n are eachnumbers, the sum of which is from 2 to 7 with at least 10 equivalents ofliquid ammonia at a temperature of from -60° C. to -33° C. for from 0.1to 24 hours to yield a perfluorodiamidine and unreacted ammonia; (b)removing at least 99% of the unreacted ammonia to yield a purifiedperfluorodiamidine; (c) reacting said purified perfluorodiamidine withfrom 1.5 to 2.5 equivalents of said original perfluorodinitrile oranother perfluorodinitrile selected from the same group of materials for1 to 72 hours at 0° C. to 70° C. to yield aperfluoro(imidoylamidine)dinitrile; and (d) reacting saidperfluoro(imidoylamidine)dinitrile with at least 10 equivalents ofliquid ammonia for 0.1 to 24 hours at -60° C. to -33° C. to yield aperfluoro(imidoylamidine)diamidine.
 4. A process for preparing anoligomeric perfluorodinitrile which comprises(a) reacting an originalperfluorodinitrile selected from the group of materials having theformula NC--R_(f) --CH wherein R_(f) is selected from biavalentfluorocarbon radicals of the formula --(C_(p) F_(2p))--, wherein p is anumber from 2 to 18 inclusive, and bivalent oxyfluorocarbon radicals ofthe formula

    --CFY(OCF.sub.2 CFY)--O--(CF.sub.2)--O--(CFYCF.sub.2 O)--CFY--

wherein Y is F or CF₃, q is a number from 1 to 18 and m and n are eachnumbers, the sum of which is from 2 to 7 with at least 10 equivalents ofliquid ammonia at a temperature of from -60° C. to -33° C. for from 0.1to 24 hours to yield a perfluorodiamidine and unreacted ammonia; (b)removing at least 99% of the unreacted ammonia to yield a purifiedperfluorodiamidine; (c) reacting said purified perfluorodiamidine withfrom 1.5 to 2.5 equivalents of said original perfluorodinitrile oranother perfluorodinitrile selected from the same group of materials for1 to 72 hours at 0° C. to 70° C. to yield aperfluoro(imidoylamidine)dinitrile; (d) reacting saidperfluoro(imidoylamidine)dinitrile with at least 10 equivalents ofliquid ammonia for 0.1 to 24 hours at -60° C. to -33° C. to yield aperfluoro(imidoylamidine)diamidine and unreacted ammonia; (e) removingat least 99% of the unreacted ammonia to yield a purifiedperfluoro(imidoylamidine)diamidine; and (f) reacting said purifiedperfluoro(imidoylamidine)diamidine with from 1.5 to 2.5 equivalents of aperfluorodinitrile selected from the group of dinitrile materials ofstep (a) and the perfluoro(imidoylamidine)dinitrile materials asproduced in step (c) for 1 to 72 hours at 0° C. to 70° C. to yield theoligomeric perfluoro(imidoylamidine)dinitrile.
 5. A process forpreparing an oligomeric perfluorodinitrile which comprises(a) reactingan original perfluorodinitrile selected from the group of materialshaving the formula NC--R_(f) --CH wherein R_(f) is selected frombivalent fluorocarbon radicals of the formula --(C_(p) F_(2p))--,wherein p is a number from 2 to 18 inclusive, and bivalentoxyfluorocarbon radicals of the formula

    --CFY(OCF.sub.2 CFY)--O--(CF.sub.2)--O--(CFYCF.sub.2 O)--CFY--

wherein Y is F or CF₃, q is a number from 1 to 18 and m and n are eachnumbers, the sum of which is from 2 to 7 with at least 10 equivalents ofliquid ammonia at a temperature of from -60° C. to -33° C. for from 0.1to 24 hours to yield a perfluorodiamidine and unreacted ammonia; (b)removing at least 99% of the unreacted ammonia to yield a purifiedperfluorodiamidine; (c) reacting said purified perfluorodiamidine withfrom 1.5 to 2.5 equivalents of said original perfluorodinitrile for 1 to72 hours at 0° C. to 70° C. to yield aperfluoro(imidoylamidine)dinitrile; (d) reacting saidperfluoro(imidoylamidine)dinitrile with at least 10 equivalents ofliquid ammonia for 0.1 to 24 hours at -60° C. to -33° C. to yield aperfluoro(imidoylamidine)diamidine plus unreacted ammonia product; (e)repeating steps (b), (c) on the product of step (d) to yield anoligomeric dinitrile; and (f) repeating steps (d) and (c) on theoligomeric product of step (e) until said product has a desiredmolecular size.
 6. The process of claim 4 wherein saidperfluorodinitrile is selected from said original perfluorodinitrile, asecond perfluorodinitrile and said perfluoro(imidoylamidine)dinitrile.